Follow-up control system



Sept. 17, 1946. s. GODET FOLLOW-UP CONTROL SYSTEM Filed Nov. 30, 1943F`ig2.

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Patented Sept. 17, 1946 UNITED STATES FLLOW-UP CGNTROL SYSTEM SidneyGodet, Albany, N. Y., assigner to General Electric Company, acorporation of New York 2 Claims.

rIhis invention relates to control systems, more particularlyl tofollow-up control systems, and it has for an object the provision of asimple, reliable, and improved control system of this character.

More specifically, this invention relates to follow-up control systemsin Which Selsyn systems are used as indicators of system error, i. e.,positional disagreement of the pilot device and driven object, tocontrol the driving means to drive the driven object into correspondencewith the pilot device. In certain of these follow-up systems, a lowspeed Selsyn system is provided which exercises a coarse control overthe driving means when the error exceeds a predetermined value, and ahigh speed Selsyn system is provided for exercising a ne highly accuratecontrol when the error is less than this predetermined value. Meansresponsive to a predetermined magnitude of the coarse control voltageare provided for transferring the control of the driving means Yfrom oneto the other of these iine and coarse control means as the error becomesgreater or less than the predetermined value.

The high speed and low speed Selsyn systems produce alternating voltagesof which the effective values vary sinusoidally with the magnitude ofthe error; varying from Zero at zero error to a positive maximum at 90degrees rotation of the Selsyn, Zero at 180 degrees, negative maximum at270 degrees, and zero at 36()` degrees. The phase of this voltagereverses at sero and 180 degrees. The control is so designed that thedirection of rotation oi the driving means which drives the drivenobject depends upon the phase of the control voltage. Consequently, Whenthe error is less than 180 degrees, the driving means operates in thedirection to drive the driven object toward correspondence with thepilot device by the shortest path, and when the error exceeds 180degrees, the driving means operates in the reverse direction. Thus, zeroerror is a point of stable equilibrium for a Selsyn system which has agearing ratio of 1:1 with respect to the driven object; the ISG-degreeerror peint is a point or unstable equilibrium. That is to say, thesystem can come to rest With the driven object exactly 180 degrees outof phase with the pilot device. However', if this error is increased ordecreased in the slightest degree, the phase of the resultant controlvoltage produced by the one speed Selsyn system will be such as toenergize the driving means for operation in the direction to drive thedriven object toward zero error or correspondence with the pilot device.

In order that the stable positions of the fine and coarse systems shouldcoincide, the high and iov7 speed Selsyn systems Were aligned so thatthe voltages produced by both Selsyn systems were in phase with eachother Within a predetermined Zone on either side of Zero error. When theratio between the high and low speed Selsyn systems is an even number,which frequently it is required to be, the voltages produced by the highand low speed Selsyn systems are of opposite phasey Within apredetermined zone on either side of the M50-degree error point. Withinthis Zone, the coarse control voltage is less than the predeterminedvalue at which the control is transferred to the fine controlling means.If an attempt is made to synchronize the system from a point Within thiszone, the coarse system never takes control, and the ne system holds thedriven object at the M50-degree error point, because this is a stablepoint as iar as the fine system is concerned. In other Words, with thiseven numbered ratio between the high and low speed Selsyn systems, the18H-degree error point becomes a false point of stable equilibrium. Thiscondition is highly undesirable, since it is possible for the follow-upsystem to become synchronized at 180 degrees error and to remain sosynchronized as long as the error remains Within this predetermined zoneon either side of 180 degrecs. Accordingly, a more specific object ofthis invention is the provision of a follow-up system utilizing high andlow speed Selsyn error indicating systems in which false points ofstable equilibrium are entirely eliminated.

In carrying the invention into effect in one form thereof, high and lowspeed Selsyn systems are provided for detecting the system error of afollow-up system and effecting a fine, highly accurate control of thedriving means at small errors and a coarse control at large errors. Theratio of the driving connections between the high and low speed Selsynsystems is an even number. The transmitter and receiver regulator of thelow speed Selsyn system are initially misaligned by approximately /11,degrees in which n is the even numbered ratio between the high and lovvspeed Selsyn systems. As a result of this misalignment of the low speedSelsyn system, the cyclically varying eiiective value oi the controlvoltage produced by the low speed Selsyn system is dephased with respectto the control voltage produced by the high speed Selsyn systemapproximately one-quarter cycle of the high speed Selsyn controlvoltage, so that at Zero error, the 10W speed Selsyn system produces avoltage Which i is proportional to the misalignment of the high and lowspeed Selsyn systems. To neutralize this zero error voltage, analternating voltage of fixed magnitude equal to the low speed Selsynvoltage at zero error and opposite in phase is added to the output ofthe low speed Selsyn system. As a result, the unstable zero for thecoarse system becomes an unstable zero for the fine system also.

For a better and more complete understanding of the invention, referenceshould now be had to the following specification and to the accompanyingdrawing of which Fig. l is a simple, diagrammatical sketch of anembodiment of the invention, and Fig. 2 is a chart of characteristiccurves which facilitate understanding of the invention.

Referring now to the drawing, an object lil is to be driven inpositional agreement with a pilot or control device ll by suitabledriving means such, for example, as represented by the direct currentmotor l2 to the drive shaft of which the object IG is connected by meansof suitable reduction gearing (not shown). Direct current is supplied tothe armature of the motor l2 by means of a generator lil having a pairof short circuited armature brushes i3d and a pair of load brushes ISDto which the armature of the motor I 2 is connected by means ofconductors I4. The generator i3 is an armature reaction exciteddynamoelectric machine and is driven at a speed which is preferablysubstantially constant, by any suitable driving means such as aninduction motor I5, to the drive shaft of which the armature reactionmachine is connected by suitable coupling means (not shown). The axis ofthe flux which is produced by the short circuited armature brushes isreferred to as the short circuit axis, and the axis which is displaced90 electrical degrees from the short circuit axis is referred to as thecontrol axis. The net flux along the co-ntrol axis is produced by thetwo opposing control field windings l3c and i3d, a series compenstingfield winding |3e, and the armature reaction of the load current whichflows through the load brushes |3'b. This net control axis flux producesthe voltage at the brushes i3d which causes current to flow in the shortcircuit, and the ilux along the short circuit axis, which is produced bythe short circuit current, produces the voltage at the load brushes |319which causes load current to flow. The important characteristics ofdynamoelectric machine i3 are its high speed of response and itsexceptionally high amplification factor, i. e., the ratio between theelectrical power supplied to the control eld winding and the electricalpower delivered at the load brushes of the machine.

The control field windings |30 and i3d on the control axis of themachine I3 are connected in the cathode-anode circuits of a single stageelectric valve amplifier which comprises the two electric valves IE5 andI1. Although these valves may be of any suitable type, they arepreferably beam power amplifier valves. As shown, they are connected forduplex operation and are provided with a self-biasing resistor l. Thecathodeanode circuits of these valves are connected in series with thesecondary windings lila and b of a supply transformer I9 of which theprimary winding ld is connected to a suitable source of alternatingvoltage, such as represented by the two supply lines 2U.

The cathode grid, or input, circuit of the amplier extends from thecathodes la and I 'la of the valves I6 and Il through the self-biasingCTI resistor I8 and the ground connection to the center tap of aresistor 2| through opposite halves of resistor 2i and the secondarywindings 22a and 22h of the grid control transformer 22 in parallel andresistors 23a, 23h, and 23e, and resistors 24a, 24h, and 24C in parallelto the control grids Ilib and |119, respectively,

With zero voltage applied to the grids Ib and Hb from the transformersecondaries 22a and 22h, the valves IB and l1 will supply circulatingcurrent through the two opposing control iield windings |30 and i3d. Themagnitude of these circulating currents is controlled as desired byadjustment of the self-biasing resistor` I8. This resistor is usuallyadjusted for half the saturation current of the Valve. The circuit isaccu'- rately balanced so that both valves normally conduct equalamounts of current. Since the control field windings l3c and l 3d opposeeach other and are equally excited when no voltage is supplied to thegrids ISD and I 'ib from the transformer 22, the net excitation ofdynamoelectric machine i3 is Zero. As a result, Zero voltage is suppliedto the motor l2 and the motor is therefore at standstill. This conditionof equal conduction in both valves occurs when the follow-up system isin correspondence, i e., when the driven object is in positionalagreement with the pilot device.

For the purpose of controlling the conduction of the valves IS and IT inaccordance with the error between the driven object and the pilotdevice, a voltage of variable magnitude is supplied to the grid circuitssubstantially in phase with the anode voltage through the transformer 22whose secondary windings 22a and 22h are connected to the grid circuitsof the valves I6 and Il, as explained in the foregoing, and whoseprimary winding is connected to the single phase alternating currentsource 25 through rotary induction apparatus illustrated as comprising arotary induction device 26 referred to as the transmitter, and a similarrotary induction device 21 referred to as the receiver regulator. Therotary induction device 26 comprises a rotor member 25a provided with asingle phase winding (not shown) and a stator member 2Gb provided with adistributed three-element winding (not shown) that is physically similarto the polyphase winding of an ordinary wound-rotor induction motor. Thestator and rotor windings are arranged in inductive relationship witheach other so that the alternating magnetic field produced by thecurrent flowing in the primary winding induces voltages in the elementsof the secondary winding. The receiver regulator is similar to thetransmitter 26 and the terminals of its stator winding are connected tothe terminals of the stator winding of the transmitter by means ofconductors 23 so that the voltages induced in the stator winding of thetransmitter cause currents to flow in the stator winding of the receiverregulator, thereby producing a magnetic field similar to the magneticfield produced by the rotor winding of the transmitter. Rotation of therotor of the transmitter causes a voltage to be induced in the rotorwinding of the receiver regulator owing to the shift in the position ofthe axis of the magnetic field of the receiver regulator relative to theaxis winding of the rotor member, and the magnitude of this inducedvoltage depends upon the relationship of the axis of this winding to theaxis of the magnetic field. When the axes of the magnetic eld and therotor winding are parallel, the induced voltage is maximum whereas whenthese axesr are at right angles with earch other,v the induced voltageis zero. It will therefore be clear that the rotation of the rotor ofthe transmitterl or of the receiver regulator will vary the magnitude ofthe voltage supplied to the grid circuit of the electric valveapparatus, which in turn, will result in a variation of the relationshipof the current ilovving in the conducting paths of the valves i6 and I1.

The grid connections from the secondary windings 22a and 2lb to thegrids Ib and Il-b are such that the voltages suppliedv to the grids are180 degrees out of phase with each other. Thus when the voltage suppliedto one of the grids increases positively, the voltage of the other gridis simultaneously made correspondingly less positive or more negative.

The rotor of `the transmitter 26 is mechanically coupled throughsuitable gearing (not shown) to the movable element of the pilot deviceii. For the purpose of increasing the accuracy and sensitivity of thecontrol, the ratio of this gearing between the pilot device and therotor oi the transmitter can be made as large as is desired. Forexample, the ratio may be 12:1, i. ef, for each degree that the pilotdevice is rotated, the rotor of the transmitter is rotated l2 degrees.The rotor of the receiver regulator 2'! is connected either to the shaftof the driving motor l2 or to the shaft of the driven object la by meansVof suitable gearing (not shown) having the same ratio as the gearingbetween the pilot device and the transmitter.

This large gear ratio provides a very rlne and very accurate control. Ifthe ratio is 121i, as assumed, then for each 30 degrees of rotation ofthe pilot device, the rotor of the transmitter 2li is rotated a full 360degrees. However, since the axes of the rotor winding of the receiverregulator 21 and the magnetic iield of the stator are parallel at twopoints in each complete revolution of the transmitter, i. e., at Zerodegrees revolution and at 180 degrees revolution of the transmitter, itwill be clear that the pilot device and the driven object must not beallowed to become more than l5 degrees out of correspondence with eachother while under the control of the high speed iine control system,because when this amount of positional disagreement occurs, the samerelationship exists between the rotors or the transmitter and receiveregulator as exists when the pilot device and driven object are incorrespondence with each other. Before power is turned on, the amount ofthis positional disagreement may be anything up to 180 degrees. Acoarser system is, therefore, provided for taking over the control fromthe high speed line control system when this amount of positionaldisagreement (l5 degrees)` is exceeded. This coarse system isillustrated as comprising a transmitter 29 that is identical with thetransmitter 26 and a receiver regulator 3i] that is identical with thereceiver regulator 2l. The single phase rotor winding of the transmitter29 is connected to the alternating voltage source 25, and the singlephase rotor winding of the receiver regulator is connected to theterminals of the primary winding of a transformer 3l, the terminals ofthe secondary winding SIb of which are connected to the grids l6b and11b through electric valves 32 and 33. The stator windings of thetransmitter 29 and the receiver regulator 30 are connected to each otherby means of conductor 34.

The rotor of the transmitter 29 is directly `connected to the rotatablemember of the pilot devicev able gearing (not shown) having the sameratio tothe driven object I0. Thus it will be seen that thetransmitter29 and the receiver regulator 3D constitute a low speed system andprovide the desired coarse control.

The electric valves 32 and 33 may be of any suitable type but arepreferably of the two-electrod-e type into the envelopes of which asmall quantity of an inert gas, such for example as neon, is introduced.A characteristic of a valve of this character is that when a voltage ofless than a predetermined critical value is applied to its terminals,the valve does not conduct current, and that when this criticalvoltageis exceeded, the neongas becomes ionized and the valve be-l comesconducting.

The transformer 3| is so designed that when the system error of thepilot device and driven object is less than a predetermined amount, e.g., seven degrees or less, the voltage applied to the valves 372 and 33is less than the ionization or breakdown voltage of these valves butequals or exceeds the ionization voltage when the system error equals orexceeds this predetermined amount. Thus, when the system error is lessthan this predetermined amount, the control connections between thecoarse control system and the grid-s lil-band l'b are interrupted, andthe coarse control system is ineffective. Conversely, when the errorequals or exceeds this amount, the valves 32' and 33' become conductingand the voltage induced in the secondary winding of the transformer 3i'is applied to the grids lb and llband thereafter effective incontrolling the valves Iii. and il. The high ohmic resistors 23a, 23h,and Esa and 26h assist the valves 32 and 33 in transferring the controlfrom the line control system tothe coarse control system when the errorequals or exceeds the predetermined amount mentionedin the foregoingdescription- The error voltage supplied from the receiver regulator ofthe high speed line control system tothe grid transformer 2?. is analternating voltage having the same frequency as that of the source 25.A plot of the eective or R. M. S. values only of this error voltage isillustrated by the sinusoidal curve 35 in Fig. 2 of which the ordinatesrepresent voltage and abscissae represent system error. Thus at Zeroerror or correspondence, the axes ot the rotor winding of the receiverregulator and of the magnetic field of the primary winding are at rightangles, and the magnitude of the error voltage is zero. Ii the error isincreased to 'l1/2 degrees clockwise, i, e., the pilot device il isadvanced '7l/2 degrees clockwise with respect to the driven object, thedisplacement of the axes of the magnetic field and of the rotor windingis increased degrees so that they are now parallel and the error voltageattains a maximum value. This error voltage is in phase with the voltageof the source The inphase relationship is indicated by the position ofthis portion of the curve 32's above the zero axis.

A further increase of the error to 15 degrees clockwise increases thedisplacement of the axes of the rotor winding and the magnetic eld ofthe stator winding another 90 degrees so that these axes are again atright angles with each other but displaced v degrees from their originalpositional relationship. Consequently, the error voltage is reduced tozero.

If the error is increased beyond 15 degrees clockwise, the phase of theerror voltage will be reversed, and this condition is indicated by theposition of the portion of the curve 35 between 15 degrees error and 30degrees error below the zero axis. Thus the amplitude of curve 35represents the magnitude of the effective value of the error voltage,and positive values of this curve indicate that the voltage is in phasewith the voltage of the source 25, and negative values indicate a18S-degree out-of-phase relationship. As indicated, the phase of thisvoltage reverses for each 15 degrees of error.

The error voltage supplied by the receiver regulator 30 of the low speedcoarse control system is also an alternating voltage having the samefrequency as that of the source 25. A plo-t of the effective values ofthis low speed Selsyn voltage is represented by the curve 36 of Fig. 2.Since the gearing ratio of the low speed coarse control system is 1:1,the error voltage is zero at zero error, maximum at 90 degrees error,and zero again at 180 degrees error. It is in phase with the voltage ofthe source 25 from zero degrees error to 180 degrees error clockwise andit is 180 degrees out of phase from 180 degrees error clockwise to zeroerror. In other words, the phase reverses at the zero-degree andLBO-degree error points.

It will be noted that within a zone l5 degrees either side of the18o-degree error point, the voltages produced by the ne and coarseSelsyn systems are 180 degrees out of phase with each other. This isindicated in Fig. 2 by the positioning of the curves 35 and 35 onopposite sides of the zero axis Within the -degree error zone on eitherside of 180 degrees error. Consequently, as long as the voltage from thetransformer 3| of the low speed coarse control system, as representedfby curve 3G, is greater than the value represented by horizontal lines3l and 38, at which the control is transferred between the ne and coarsecontrol systems, the driving motor I2 is energized for rotation in adirection to drive the driven object toward the position of zero erroror correspondence with. the pilot device. However, when this voltage isbelow the critical value represented by lines 3l and 38, at the time ofsynchronization the voltage produced `by the fine control system whichis of reverse phase with respect to the voltage from the coarse controlsystem will energize the motor i2 to drive the object lo in the reversedirection. In other words, the motor will be energized to drive theobject SG toward the 180- degree error point. If, while the power isremoved from the system, the pilot device il is moved out ofcorrespondence an amount such that the error of the system falls withina zone of approximately 71/2 degrees on either side of the 180-degreeerror point within which zone the coarse control voltage is less thanthe critical voltage, and which zone is represented by the verticallines 39 and 40, the driven object lil will be synchronized 180 degreesout of correspondence with the pilot device when the power is restoredto the system. Thisv operating condition is highly objectionable and itis therefore desirable to eliminate this zone of stable equilibriumdescribed in the foregoing.

For the purpose of eliminating this point of stable equilibrium, thetransmitter 23 and receiver regulator 33 of the coarse control systemare misaligned `by approximately 90/11 degrees so that the outputvoltage of the coarse system is dephased by this amount, orapproximately onequarter cycle of the output voltage of the iine controlsystem. This dephased voltage is represented by the dotted sinusoidalcurve 4l in Fig. 2. This curve crosses the zero error axis at point dla.The ordinate of point 4Ia is therefore a measure of the voltage producedby the coarse control system at zero error. Such a voltage at zero errorwould tend to synchronize the system at an error corresponding to thezero point 4Ib of the dephased voltage curve 4|. To eliminate thistendency, a iixed voltage equal in magnitude to the dephased voltage ofthe coarse control system at the zero error and of opposite phase isadded to the output of the receiver regulator 3l). This voltage isderived from a secondary Winding |3c of transformer I9. The secondarywinding is connected in series with the output winding of receiverregulator 3U and the primary winding 3io of transformer 3| by means ofconductors 42. The polarity of the connections of the secondary windingI 9c in this circuit are such that the phase of the added voltage isopposite to that of the output voltage of the receiver regulator 38 atzero error. The resultant of the two voltages is represented by thesinusoidal curve 43 in Fig. 2. This curve passes through the point ofzero error and zero voltage, i. e., the voltage is zero at the Zeroerror.

The voltage represented by curve 43 also passes through zero at point43a which, owing to the ratio of 12:1 between the high and low speedSelsyn systems which has been assumed, occurs at approximately degreeserror clockwise. IThe voltage represented by curve 43 intersects thelines 31 and 38 which represent the critical voltage at points 43h and43e, respectively. These points 4.3i: and 43e define a zone on eitherside of the zero voltage point 43a within which the output voltage ofthe slow speed Selsyn system represented by curve 43 is in phase withthe output voltage of the high speed Selsyn system represented by curve35. Thus, the point 35a on the curve 35 which represents the voltage ofthe high speed Selsyn system and which corresponds to the point 43a oncurve 43 is a point of unstable equilibrium of the high speed Selsynsystem. In other words, for any error between the points 43h and 43ewithin which zone control of the driving moto-r l2 is transferred to thehigh speed Selsyn system, the voltage produced by the high speed Selsynsystem will have the same phase as the voltage produced by the low speedSelsyn system, and will therefore cause the motor l2 to drive the drivenobject toward the zero error point. Thus, the zero erro-r point remainsa point of stable equilibrium and the point 35a which corresponds to thepoint 43a of curve 43, which is the only other zero point of the curve43, is a point of unstable equilibrium. Thus, the second or false pointof stable equilibrium is eliminated. As a result, the driven object I0cannot be synchronized with the pilot device at a false point of stableequilibrium.

It is not necessary that the second zero point 43a of the voltagerepresented by curve 43 should coincide exactly with a zero point ofunstable equilibrium of the high speed Selsyn system. It is onlynecessary that no stable zero of the high speed Selsyn system occurwithin the zone dened by the points 43h and 43e within which control ofthe driving mo-tor I2 is transferred to the high speed Selsyn system.

Although in accordance with the provisions of the patent statutes thisinvention is described as embodied in concrete form and the principlethereof has been explained, together with the best mode in which it isnow contemplated applying that principle, it Will be understood that theapparatus shown and described is merely illustrative and that theinvention is not limited thereto, since alterations and modificationswill readily suggest themselves to persons skilled in the art withoutdeparting from the true spirit of this invention or from the scope ofthe annexed claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. A follow-up control system comprising in combination, a pilot device,a driven object, driving means for said object, coarse and finecontrolling means responsive to positional disagreernent of said pilotdevice and driven object for producing periodically varying controlvoltages for controlling said driving means to drive said object towarda position of correspondence with said pilot device, means fortransferring control of said driving means between said ne and coarsecontrolling means in response to the magnitude of said positionaldisagreement, means for dephasing the control voltage produced by saidcoarse control means a predetermined amount with respect to the other ofsaid control voltages, and means for adding to said coarse controlvoltage an alternating Voltage opposite in phase and substantially equalin magnitude to the magnitude of said dephased voltage when said pilotdevice and driven object are in positional agree'A ment.

2. A follow-up control system comprising in combination, a pilot device,a driven object, driving means for said object, coarse controlling meansfor producing a relatively small numberl and fine controlling means forproducing a relatively larger number of cycles of a cyclically varyingcontrol voltage in response to a predetermined amount of variation inthe positional disagreement of said pilot device and driven object forcontrolling said driving means to drive said object toward positionalcorrespondence with said pilot device, means for transferring control ofsaid driving means between said iine and coarse controlling means at apredetermined value of said v positional d1sagreement, means fordephasing the control voltage produced by said coarse control meansapproximately one-quarter cycle of the voltage produced by said necontrolling means,

:and means for adding to the voltage produced by said coarse controllingmeans an alternating voltage opposite in phase and substantially equalin magnitude to the magnitude of said depliased voltage when said pilotdevice and driven object are in correspondence.

SIDNEY GODET.

